Soak the beans in a large saucepan with water to cover by three inches. Boil for 2-3 minutes, cover remove from heat and let stand 1 hour. Drain and rinse beans. Put them in large pot and add celeriac, carrot, onion, bay leaf and stock or water. Heat to a boil, reduce heat, cover and simmer, stirring occasionally, for forty-five minutes. Add the salt and olive oil. There should be enough liquid to barely cover the beans. If not, add stock or water as necessary. Continue simmering uncovered until the beans are tender, about 15 more minutes. Stir occasionally. Chop greens and wash. Shake out as much water as possible, preferably in a salad spinner. Stir the lemon juice into the beans. Add the greens a handful at a time, stirring until wilted before adding another handful. When all greens have been added, simmer for five minutes. Check seasonings, adding salt and pepper as necessary.